COADSORPTION OF OXYGEN AND CHLORINE ON PT(111)

The coadsorption of oxygen and chlorine is studied on the Pt(111) surf ace, using line-of-sight desorption mass spectrometry, LEED and work f unction measurements to characterize the resulting adlayers and to det ermine the uptake and desorption kinetics. Preadsorbed chlorine reduce s the sticking probability of oxygen to a similar extent, as does an e qual concentration of adsorbed oxygen atoms. Chlorine adsorption proce eds in the whole range of possible oxygen precoverages (less than or e qual to 0.25) with a similar sticking coefficient as on the clean surf ace and leads to a high density adlayer with a total concentration of 11.3 x 10(14) (O + Cl)atoms cm(-2) Starting with a complete (2 x 2) ox ygen adlayer, the ordered structure vanishes during chlorine dosing on ly at a total coverage (chlorine plus oxygen) of 0.5. Increasing the t otal concentration further leads to the buildup of a (3 root 3 x 3 roo t 3)R30 degrees structure at 9.3 x 10(14)(O+Cl) atoms cm(-2). Temperat ure-programmed desorption yields oxygen molecules, chlorine molecules and chlorine atoms. The two adsorbed species destabilize each other, a nd the molecule peaks are shifted to lower temperatures, in the case o f oxygen up to 70 K. With high initial coverages, this trend is not co ntinued but reversed. (C) 1998 Elsevier Science B.V. All rights reserv ed.